Soil Testing & Plant Analysis
Transcript of Soil Testing & Plant Analysis
Soil Testing Lab, Virginia Tech
Soil Testing & Plant Analysis Rory Maguire and Steve Heckendorn Virginia Tech Soil Testing Lab Manager
Lab Phone: 540-231-6893 Desk Phone: 540-231-9807 Email: [email protected]
Soil Testing Lab, Virginia Tech
Soil Testing: Used to estimate nutrient availability in
the soil and to provide fertilizer and lime recommendations.
Available through Virginia Cooperative Extension and private laboratories.
Routine Soil Test Analysis -- Soil pH plus plant available levels of
P, K, Ca, Mg & various micronutrients and estimated CEC.
Soil Testing Lab, Virginia Tech
Components of Soil Testing Sample collection Analysis Interpretation Recommendations
& Reporting
Soil Testing Lab, Virginia Tech
The greatest potential for error in soil testing is in taking the sample
Soil Testing Lab, Virginia Tech
Why do we need to collect a good soil sample?
A half to one pound sample must represent, on the average, 2 million lbs of soil per acre being sampled
MANMH: p. 157
Soil Testing Lab, Virginia Tech
Samples must be representative of the area being sampled, thus:
One sample should represent
just one management unit
Separate landscape into uniform areas no larger that 10 acres
Soil Testing Lab, Virginia Tech
Samples must be representative of the area being sampled, thus:
Take separate samples from areas that differ
significantly if they can and will be managed as different management units
Avoid areas that are not representative of the entire area, such as old fence rows and other odd spots, including areas influenced by pets, chemicals or other man-made activities
Soil Testing Lab, Virginia Tech
Soil Testing: Sampling Tools
A sampling tube or auger (trowel or spade can be used)
A clean plastic container
Laboratory’s soil sample containers
Soil Testing Lab, Virginia Tech
Samples must be representative of the area being sampled, thus: Take 10 or more subsamples
per sample area For large acreage, a minimum
of 5 cores (subsamples) per acre should be collected within a uniform area
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Sampling Depth
Turf – 2 to 4”, excluded any thatch
Gardens – 6 to 8” or tillage depth
Trees and shrubs – 6” remove any mulch or surface debris
Soil Testing Lab, Virginia Tech
Additional Guidelines: Soil Sampling
Sample before plant establishment! Areas should be tested at least once
every three years Do not wait until the last minute. Fall is a
good time to sample Recommendations are only as good as
the sample collected and information supplied.
Soil Testing Lab, Virginia Tech
“Writeable” Forms
Available Online at
www.soiltest.vt.edu
under “Fees and forms”
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Drying Samples If a soil sample is wet, then
Allow it to air-dry
Do not oven-dry
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Certified Crops Advisors Sample Exam Question The most precise component in a soil
testing program is normally: A. sampling B. laboratory analysis C. extrapolation D. interpretation and recommendations
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Soil Testing: Analysis of Samples Extractants will vary from one lab to
another!!! Using different extracts will results in
different numbers being reported for the same nutrient!
Soil Testing Lab, Virginia Tech
Selected Common Soil Test Extractants Extractant Composition Nutrient Source Mehlich I 0.05 M HCl +
0.0125 M H2SO4 P Fe/Al & Ca
bound Mehlich III 0.015 M NH4F + 0.2 M
CH3COOH + 0.25 M NH4NO3 + 0.001 M
EDTA+ 0.013 M HNO3
P Fe/Al & Ca bound
Bray P1 or Weak Bray
0.03 M NH4F + 0.025 M HCl
P Fe/Al bound
Bray P2 0.03 M NH4F + 0.1 M HCl
P
Olson 0.5 M NaHCO3 P Ca bound Ammonium Acetate
NH4OAc K Exchangeable
Soil Testing Lab, Virginia Tech
First Soil Fertility Test
50 B.C. Columella
recommended the Taste Test to
measure acidity and salinity of soils.
Soil Testing Lab, Virginia Tech
Components of Soil Testing Sample collection Analysis Interpretation – Sufficient, Low, etc.
Recommendations – Fertilizer and Lime Needs
Soil Testing Lab, Virginia Tech
P & K Recommendations: VT STL Phosphorus: L+, L, L- (<12 lb/a) – Critical Level
Potassium: L, L- (<56 lb/a) – critical value Loamy sands and sandy loams, K will
tend to leach and accumulate in the subsoil. If plant roots can reach this K, then K may not be a problem despite a low test level in the top several inches
Soil Testing Lab, Virginia Tech
Ca & Mg Recommendations: VT STL
Calcium: L- - deficient for peanuts L- - may not be deficient for other plants, but
pH is normally too low for optimum growth Magnesium: L- - critical level for coastal plain soils L-, L – critical level for Piedmont &
Appalachian soils Apply dolomitic limestone if pH is low If pH is optimum, apply 30 lbs Mg/A
Soil Testing Lab, Virginia Tech
What’s Needed to Make a Lime Recommendation
Crop Code sets Target pH - where you want to be.
Soil (water) pH tells where you are. Amount of Exchangeable/Residual Acidity
(Buffering Capacity of Soil) tells how much lime is needed to get from WpH to TpH
More Clay = CEC = Exch. Acidity
Soil Testing Lab, Virginia Tech
Lime Recommendations
.
BpH of Unlimed Soil
5.2
Target pH 6.2
6.8
------ lime, T/A ----- 6.60 0.00 0.00 0.00 6.30 0.00 0.50 1.00 6.00 5.70
1.00 2.25
2.00 3.75
2.75 4.50
5.40 3.75 5.25 6.25
VNMS&C: p. 44
Soil Testing Lab, Virginia Tech
VT Lime Recommendations are Based on the Following Factors:
1. Crop to be Grown (sets target pH)
2. Soil [water] pH (plays a small role)
3. Soil Buffer [pH] Index (measures total acidity / buffering capacity of soil)
4. Credit For Previous Lime Application
Bas
icity
A
cidi
ty
Strong
Medium
Slight Slight
Moderate
Medium Strong
Very Strong
9.0
8.0
7.0
6.0
5.0
4.0
Neutrality
Is current pH lower than the
target pH?
MANMH: p. 47
Target pH
Current pH
Soil Testing Lab, Virginia Tech
Important to Know the Method! Different buffer solutions (Initial pH):
Mehlich (6.6 pH) Woodruff (7.0 pH) SMP (7.5 pH) Sikora (7.7 pH) Adams-Evans (8.0 pH) Note that a lot of other buffer readings will be
higher than Mehlich BpH’s starting pH. So if a BpH is > 6.6, then it is probably not a Mehlich buffer value.
Soil Testing Lab, Virginia Tech
Soluble Salts Test $2 Not Normally Needed
Container Grown Band fertilizer too close to the seed Drought Salts from external source (over-fertilize, groundwater, VDOT,
cleaning chemicals) Ocean salt from storm surges and
brackish waters
Soil Testing Lab, Virginia Tech
Soil Organic Matter Test $4 Not normally needed Farmers use to adjust herbicide rates To know more precisely the amount –
to verify contract specifications To compare levels at different times Requested most on garden samples
Soil Testing Lab, Virginia Tech
Soil Components Sand
No Electrical Charge (Neutral) Silt
No Electrical Charge (Neutral) Clay
Negative Electrical Charge
Est.-CEC suggests ballpark clay content of soil.
Soil Testing Lab, Virginia Tech
“Balancing the Soil” As in the Basic Cation Saturation concept ,
is the approach a soil should contain a certain percentage of each of the basic cations, to be “balanced”.
(e.g., 65-75% Ca, 10-12% Mg, 2-5% K)
“Balancing the Soil”
• subscribes to the “sufficiency level” concept and not the “basic cation saturation” idea.
• The “Balancing” approach has not stood up well under scrutiny.
X
not
not
Soil Testing Lab, Virginia Tech
Soil Testing Lab, Virginia Tech
Common Conversions P x 2.3 = P2O5 P2O5 2.3 = P K x 1.2 = K2O K2O 1.2 = K NO3
- 4.4 = NO3-- N NO3
-- N x 4.4 = NO3-
ppm x 2 = lb/A lb/A 2 = ppm
Labs report values in different forms and units!
Soil Testing Lab, Virginia Tech
Using Results from other Soil Testing Laboratories
Results from other labs must be converted to Virginia Tech values so that recommendations can be made based on VALUES recommendations.
J. T. Sims (1989)
Soil Testing Lab, Virginia Tech
Tissue Testing Most commonly used to diagnose nutritional
problems related to poor growth or color, or to monitor the effectiveness of fertilizer practices on growing plants.
Not a substitute for soil testing. Most effective when used in conjunction with a
regular soil testing program.
Soil Testing Lab, Virginia Tech
Tissue Testing – Sample Collection Proper sampling requires that a specific
plant part be taken (particular leaf, group of leaves or portion of the plant)
Soil Testing Lab, Virginia Tech
Tissue Testing – Sample Collection If no instructions are available –
general rule of thumb is to sample the upper, most recently mature, fully developed leaf
Soil Testing Lab, Virginia Tech
Tissue Testing – Sample Collection
DO NOT COLLECT: Diseased or dead plant material Materials damaged by insects or
mechanical injury Plant that have been under nutrient stress
for an extended period of time.
If a nutrient deficiency is expected: Collect samples from affected area and
from normal plants in the immediate or adjacent areas
Soil Testing Lab, Virginia Tech
Tissue Testing
If leaves are dusty: brush or wipe with a damp cloth to
remove contaminates or Wash in a mild detergent and rinse in
running water.
Air-Dry tissue samples before shipment to the laboratory